This invention relates generally to the field of information storage, and more particularly to a method and apparatus for eliminating voltage transients or glitches in a disk drive read head, particularly while switching the biasing.
In general, mass storage devices, such as hard disk drives, include a magnetic storage media, such as rotating disks or platters, a spindle motor, read/write heads, an actuator, a pre-amplifier, a read channel, a write channel, a servo circuit, and control circuitry to control the operation of the hard disk drive and to properly interface the hard disk drive to a host system or bus.
In advanced disk drives, magneto-resistive (MR) heads are used for reading and writing data. An MR head generally includes an MR read element or sensor for reading data from the data tracks and an inductive write coil for writing data to the data tracks. In order to guide the MR head over a data track while the disk is rotating, servo sectors are placed in the track along with data sectors. These servo sectors are read by the same MR sensor that reads the data sectors. The presence of the servo sectors makes it necessary to interrupt the writing during write cycles of the MR head as the head passes over a servo sector to allow the head to read the sector. Furthermore, with the disk rotating at a very high speed, it is crucial that the MR head be ready for reading data almost immediately after it is switched from write or idle mode to read mode, to avoid skipping portions of the track. The thermal response of the MR sensor, due to the variation in the MR sensor bias current as the MR head mode changes, prevents the MR sensor from reaching steady-state until several microseconds after the MR head changes its operational state.
Pre-amplifying circuits for MR heads are described, for example, in U.S. Pat. Nos. 5,122,915 and 5,032,935. Due to the thermal response problem described above, the prior art pre-amplifying circuits typically generate undesirable transient voltages at their outputs as the MR head switches from write or idle mode to read mode. These transients result because the MR sensor is biased with a different current in each operational state.
Normally, the MR sensor is not biased when the MR head is in write mode or in idle mode to save power. With the MR head off, overdriving may occur, which is caused mainly by the capacitance coupling between the MR sensor and the write coil of the MR head. When the MR head is in write or idle mode, the amplifier coupled to the MR sensor is usually switched off. Upon the MR head switching to read mode, the MR sensor is again biased with a DC-current. This bias current causes the temperature of the MR sensor to rise from its value when the MR head was in write or idle mode until the steady-state temperature is achieved. The temperature rise results in a change in the MR sensor resistance, which depends on the temperature coefficient of the MR sensor. The variation in the MR sensor resistance in turn produces a transient voltage at the output of the read-back pre-amplifying circuit, thus preventing the MR head from properly reading data for several microseconds immediately after the MR head switches to read state.
MR heads typically use current biasing to sense the data on the disk. The current bias can be used to bias the MR heads during write mode for the benefits described above. More recently, voltage biasing has been used to supply a bias to the head during servo data write and at other times such as an idle state. Using voltage biasing during servo bank write has the advantage of insuring a more constant bias current when several or all the heads are being written simultaneously. Biasing of the MR head during servo bank write is important to protect the head as well as keep the MR head at the proper temperature. A voltage transient or glitch often occurs when the head bias is switched from current bias to voltage bias and at other times when switching is done in the pre-amp circuit. Eliminating or reducing these glitches is also important to protect the MR head and extend the head lifetime.
The present invention provides an improved bias circuit for a disk drive head which reduces or eliminates transients while switching biasing. Embodiments of the invention are specifically directed to eliminating transients while switching the bias of a MR head such as from current bias to voltage biasing.
In an embodiment of the present invention, bias enable signals from a control circuit are inputs to delay circuits. The delay circuits provide a delay on the high-to-low transition, and essentially no delay on the low-to-high transition. This unsymmetrical delay ensures that the read head bias current will continue to be driven during the biasing transition to reduce voltage swings that could damage the head.